I tried to melt Hersheys sugar free choc chips with the double boiler techique. It finally melted after becoming a lump of fudge and by adding a bit of canola oil and a drizzle of 2% milk. Now it doesn't want to harden. What have I done to myself here?

The addition of the canola oil to help melt the chocolate is the likely problem. The freezing/solidification point of canola oil is -10degrees C. Try using coconut oil which has a freezing/solidification point of 25 degrees C. The coconut oil will be meted before the chocolate chips and will help them melt, and once melted and thoroughly stirred in, should enable the mixture to re-solidify. Alternatively, you could also use a bit of butter or crisco shortening which is more solid at room temperature in place of canola oil.